1. Field of the Invention
The present invention relates to a variable phase shifter for use in the microwave and millimeter-wave regions. This invention is particularly useful for applications such as phased array antennas.
2. Description of the Related Art
In a phased array antenna, a variable phase shifter is used to introduce a fixed amount of phase shift to signals carried on a transmission line. Conventional phase shifters are of two types, i.e., a switched line type and a loaded line type. The switched line variable phase shifter comprises two line segments of different length selectively connected to the transmission line and the differential path length between them determines the amount of phase shift to be introduced. The transmission line is switched over from one line segment of the phase shifter to the other when a phase shift is introduced and switched back to the original line segment when the phase shift is removed. If the intended phase shift is 180 degrees, the differential path length is equal to the half wavelength of the center frequency of the signal on the transmission line. However, the phase shift value tends to deviate linearly from the intended value as the frequency of the signal deviates in either direction from the center (nominal) frequency. To maintain the phase deviation in a small range, it is necessary to control the frequency of the signal to within narrow limits.
The loaded line phase shifter comprises a pair of spaced-apart main stubs extending from the transmission line and a pair of extension stubs selectively coupled to the main stubs. The phase shifter of this type also suffers from the frequency-dependent phase deviation. Specifically, the characteristic of the loaded line type has a minimum phase deviation at the center frequency. If the circuit layout is optimized, there exist a nonlinear negative phase deviation from the intended value that increases as the frequency decreases in a direction away from the center frequency and a nonlinear positive phase deviation from the intended value that increases as the frequency increases in a direction away from the center frequency. If the circuits are laid down perfectly, the negative and positive phase variations follow a curve that is symmetrical with respect to the nominal frequency. However, due to inherent imperfections, the characteristic curve loses the symmetricity and the phase shift value deviates from the intended value with frequency deviation of the signal from the nominal value, in a manner similar to the switched line phase shifter.
It is therefore an object of the present invention to provide a variable phase circuit in which the frequency-dependent phase deviation is compensated.
According to a broader aspect, the present invention provides a variable phase circuit comprising a transmission line, a phase shifter for introducing a phase shift to and removing the phase shift from the transmission line, an open-ended loading stub, and a switch for connecting the open-ended loading stub to and disconnecting the loading stub from the transmission line according to operation of the phase shifter.
The open-ended loading stub has the effect of cancelling phase deviation of the signal as its frequency deviates from nominal (center) frequency.
According to a second aspect of the present invention, there is provided a variable phase circuit comprising a transmission line, a plurality of phase shifters respectively responsive to control signals for introducing one of a plurality of different phase shifts to and removing the introduced phase shift from the transmission line, a plurality of loading stubs, and a plurality of switches for selectively connecting the loading stubs to and disconnecting the loading stubs from the transmission line in accordance with operation of the phase shifters.